A protein with a molecular weight of approximately 370,000 has been identified in the duodenal mucosa of rats which binds iron, lead, cobalt and calcium both in vivo and in vitro. This mucosal metal binding protein is heat stable at 60 degrees C for ten minutes and appears immunologically distinct from ferritin, transferrin and lactoferrin. Further, its binding of each of the divalent metals cited above both in vivo and in vitro demonstrates properties which are different than the previously known iron binding proteins. The mucosal metal binding protein has a greater avidity for iron than lead which probably explains why carrier iron inhibits the absorption of lead but the converse is not observed. The increased absorption of lead in iron deficient rats and diminished absorption in iron loaded animals is postulated to be due to a saturation of the mucosal protein with iron so that absorptive sites for lead are either increased or decreased in the various states of iron repletion. This would provide a mechanism for the prevalence of lead intoxication in iron deficient children. Pure preparations of the mucosal metal binding protein are being obtained for characterization and the development of antibodies which will permit an improved understanding of the regulation of iron absorption and other divalent metals.